Method for washing pulp in a bleaching line

ABSTRACT

The method is for bleaching cellulose pulp in a bleaching line that has at least two bleaching steps including a first (D 1 ) and a second (D 2 ) bleaching step. The bleaching steps have wash apparatuses (W 4 , W 5 ) for the pulp arranged after the first and the second bleaching step. Wash liquor and where appropriate dilution liquor is led in principle in counter-current to the pulp flow through the bleaching steps in the bleach line (W 1 -D 0 -W 2 -EO/EOP-W 3 -D 1 -W 4 -D 2 -W 5 ). At least one of wash liquor and dilution liquor is taken to the subsequent wash (W 5 ) of the second bleaching step (D 2 ) and at least a part of the wash filtrate from the subsequent wash of the second bleaching step is led to a second branch position (A 2 ) in the main conduit.

The present invention relates to a method of bleaching cellulose pulpand a bleach line for the method.

PRIOR ART

It is a desire in multi-stage bleaching of cellulose pulp to reduce thewater requirement and the quantity of outlet contaminated process water,which contaminated process water is either pumped to sewage, possiblyvia sedimentation basins and/or to costly destruction/deposition.

With the object of reducing the liquor quantities, the bleachingdepartment is ever more closed and most often the process water is leadin counter-current to the direction of flow of the cellulose pulp in theprocess steps of the bleaching line. Accordingly, the fresh water or theclean process water is used in the wash of the last step and the washfiltrate obtained there from is led as wash liquor to the wash of thepreceding step and further up through the bleaching line.

Commonly, a number of filtrate tanks are used between each bleachingstep wash for this leading of wash filtrate through the process, withthe object of guaranteeing supply of wash liquor and securing that thewash liquor is lead in counter-current to the flow of cellulose pulp.

In addition to such filtrate tanks, expensive control and regulationsystems with valves are required for this handling of the wash liquor infiltrate tanks, to monitor the levels in the filtrate tanks since therisk can not be taken that a filtrate tank is emptied whereby it maycause a stoppage in the wash of the bleaching step in question.

The number or filtrate tanks also results in a risk of an increasedoutlet of odorous gases as all filtrate tanks require ventilation inorder to level out changes in the volume in the filtrate tanks. Often,special degassing systems are required to handle and destruct suchodorous gases.

Accordingly, big advantages could be attained if the number of filtratetanks between the process steps could be minimised.

THE OBJECT AND PURPOSE OF THE INVENTION

One object of the invention is to reduce the need of, and in some casescompletely eliminate, such expensive filtrate tanks, control systems andvalves in the wash liquor systems, whereby the investment costs for thebleaching line may be strongly reduced. The reduced number of necessaryfiltrate tanks also results in the possibility of a more compact andmore optimal design of the bleaching line, without consideration of suchfiltrate tanks that conventionally numbers to at least the same numberas the number of bleaching steps, with a more efficient layout of thebleaching steps of the bleaching line.

Yet another object is to increase the runnability/accessability of thesystem as several control valves may be removed, that otherwise arealways potential risks for plugging/stoppage of the liquor distributionsystem.

Yet another object is to improve the runnability as the risk ofmixing-in of air in the filtrate system is considerably reduced when thenumber of filtrate tanks can be considerably reduced. At the same time,accumulation of floating pulp is avoided, which floating pulp usuallyaccumulates after a certain running time, by surface flotation infiltrate tanks. Principally in alkaline steps, such accumulated floatingpulp may rise to a level of a few metres above the surface in thefiltrate tanks and it must be continuously taken care of or recycled tothe bleaching line in order not to risk plugging of the filtratesystems.

Yet another object is that the bleaching plant can be rendered moreenvironmentally friendly as occasional overloads in certain positions,so called over-runs, need not result in outlet of gas or liquor.

Yet another object is to minimise the water consumption.

By the system, the system itself may compensate for occasional changesin wash liquor requirements in the various bleaching steps and securethat a required wash liquor quantity is always guaranteed the bleachingsteps.

It is yet another object to minimise the energy consumption in pumps inthe filtrate distribution system, where instead a pressurised filtratemain conduit is maintained and any required liquor quantity is drawn offform the main conduit, as needed.

Yet another object is to decrease the length of the tube system, whichreduces the costs of installation and the complexity of the system,whereby in the latter case the lucidity is also increased for theoperators.

A cost reduction of between 1 and 2 millions USD can be obtained for a4-step bleaching line D₀-EOP-D₁-D₂ with intermediate wash steps, if theinvention is fully applied.

LIST OF DRAWINGS

FIG. 1 shows a conventional prior art bleaching sequence D₀-EOP-D₁-D₂ inwhich the filtrate is led in counter-current, via filtrate tanks;

FIG. 2 shows the same bleaching sequence D0-EOP-D1-D2 in which thefiltrate is lead between the steps in accordance with the invention,according to an embodiment using a high pressure mode,

FIG. 3 shows a part of the bleaching sequence as also shown in FIG. 2,but according to another embodiment of the invention where a lowpressure mode is used, and,

FIG. 4 shows a section of a further embodiment according to any of thebleaching sequences according to FIG. 2 or FIG. 3, wherein minormodifications have been performed.

PRIOR ART

FIG. 1 shows a conventional bleaching sequence D₀-EOP-D₁-D₂ in which thefiltrate from the washing between the steps is led in counter-currentbetween the bleaching steps, via filtrate tanks FT₁-FT₄.

The pulp is pumped by a pump from a first storage tower, to a first washW₁ in which the pulp is washed with a clean first filtrate FF1. In thefigure, wash apparatuses of wash press type are schematically shown,having two contra-rotating wash drums where wash liquor is supplied tothe web of pulp on both drums, but subsequently the figure only showsthe supply of wash liquor to one drum of the wash press.

It is typical to such bleaching systems having wash presses that thebleaching takes place in reactors at a pulp consistency of 10-14% andthat after treatment in the reactor, the pulp is diluted to about 5-10%,typically about 8%, before it is fed to the wash press. After the washpress, the pulp has a consistency of 20-35%, typically 30%. After thefirst wash W₁, the washed pulp is fed down into a chute in which thepulp is diluted by a liquor that is pumped from a filtrate tank FT₁,from which chute it is pumped by a pump and a subsequent mixer M1 to asubsequent bleaching step, here a first chlorine dioxide step D₀ shownas an up-flow tower (the pulp flows upwards in the tower). The chemicalsfor the bleaching step, ClO₂ and acidifier H₂SO₄, are mixed-in by themixer M1 before the pulp is led to the D₀ bleaching tower.

After the bleaching in the D₀ bleaching tower, the pulp is led to achute in which the pulp is diluted by filtrate from the first filtratetank FT₁. From the chute, the pulp is pumped to a subsequent wash W₂ inwhich the pulp is washed by wash liquor from a third filtrate tank FT₃.

Thereafter, the pulp washed in the wash W₂ is led to a chute in whichthe pulp is diluted by filtrate from a second filtrate tank FT₂, andfrom the chute the pulp is pumped by a pump and a subsequent mixer M2 toa subsequent alkaline extraction step, here an EOP step shown as anup-flow tower. The chemicals for the extraction step, NaOH and peroxideH₂O₂, and oxygen gas if needed, are mixed in by the mixer M2 before thepulp is led to the EOP tower.

After the treatment in the extraction tower EOP, the pulp is led to achute in which the pulp is diluted by filtrate from the second filtratetank FT₂, where after the pulp is pumped to a subsequent wash W₃. In thewash W₃, the pulp is washed by clean filtrate FF2.

After the wash W₃, the washed pulp is fed down to a chute in which thepulp is diluted by filtrate from a third filtrate tank FT₃, where afterthe pulp is pumped by a pump and a subsequent mixer to a subsequentbleaching step, here a second chlorine dioxide step D₁, shown as anup-flow tower. The chemicals for the D₁ bleaching step, ClO₂ andpH-adjuster, are mixed in by the mixer before the pulp is led to the D₁bleaching tower. As an alternative, the adjusting of pH can take placeby addition of e.g. NaOH in the preceding chute.

After the treatment in the D₁ bleaching tower, the pulp is led to achute in which the pulp is diluted by filtrate from the third filtratetank FT₃, where after the pulp is pumped to a subsequent wash W₄. In thewash W₄, the pulp is washed by filtrate from a fourth filtrate tank FT₄.

After the wash W₄, the washed pulp is fed down to a chute in which thepulp is diluted by filtrate from a fourth filtrate tank FT₄, where afterthe pulp is pumped by a pump and a subsequent mixer to a subsequentbleaching step, here a third chlorine dioxide step D₂, shown as anup-flow tower. The chemicals for the D₂ bleaching step, ClO₂ andpH-adjuster, are mixed in by the mixer before the pulp is led to the D₂bleaching tower. As an alternative, the adjusting of pH can take placeby addition of e.g. NaOH in the preceding chute.

After the treatment in the D₂ bleaching tower, the pulp is led to achute in which the pulp is diluted by filtrate from the fourth filtratetank FT₄, where after the pulp is pumped to a subsequent wash W₅. In thewash W₅, the pulp is washed by clean filtrate FF3.

The pulp bleached by the shown bleaching sequence, D₀-EOP-D₁-D₂, issubsequently led to a storage tower (not shown) and typically has abrightness above ISO 80 and is often a fully bleached pulp of ISO 90. Incertain cases, a subsequent treatment can be used to modify theproperties of the pulp in respect of drainage properties etc. The mainprinciple of the filtrate distribution of the shown bleaching sequenceis that there are filtrate tanks between the treatment steps, whichfiltrate tanks receive the filtrate from the wash in question.

The filtrate tank FT₄ of the last wash W₅ collects the filtrate and thenthe filtrate is led in counter-current to the flow of pulp through thebleaching line, via pumps, and is used as dilution or wash liquor inpreceding positions. In a corresponding manner, filtrate from the washapparatuses W₄, W₃, W₂ is collected in the filtrate tanks FT₃, FT₂ andFT₁, respectively, and then the filtrate is led via pumps, from therespective tank in counter-current to the flow of pulp through thebleaching line.

In certain circulations, a certain share of the filtrate is also bledoff, as is shown in the feed from the filtrate tanks FT₁ and FT₂, inorder to avoid accumulation of increasing contents of undesiredsubstances, which bleeding-off is compensated by supply of cleanerfiltrates FF1 and FF2. The bleeding-off of filtrate is the principleoutlet from the bleaching line. In this counter-current filtratedistribution, alkaline filtrate is separated from acidic. Accordingly,the alkaline filtrate from the EOP step is collected in the filtratetank FT₂, and no acidic filtrate is used in the wash W₃, but insteadclean filtrate FF2 is used. In certain applications, such alkalinefiltrate can be fed on, upstream, to the oxygen delignification, whereit is used as wash liquor in the wash after the oxygen delignification.

For the acidic filtrates that are collected in the filtrate tanks FT₄,FT₃ and FT₁, the wash liquor is led strictly counter-current to the flowof pulp, i.e. from FT₄ to FT₃, and finally to FT₁ from where the acidicfiltrate is bled off from the bleaching department since it can not behandled in the recovery system, mainly due to high contents of chloridethat destroy the soda recovery boiler.

PREFERRED EMBODIMENT OF THE INVENTION

FIG. 2 shows an embodiment of the invention, in which the same bleachingsequence is performed as described/shown in relation to FIG. 1, but inwhich the filtrate distribution system instead of the large number offiltrate tanks, has been replaced by a joint main conduit 1 for allacidic bleaching steps, in accordance with the invention.

Accordingly it is understood that according to the invention bleachingof the cellulose pulp takes place in a bleaching line with at least twobleaching steps in the bleaching line and at some point including afirst and a second bleaching step D₁, D₂ in succession, as seen in theflow direction of the cellulose pulp, which bleaching steps have washapparatuses W₄ and W₅ for the pulp arranged after the first and thesecond bleaching step, respectively. As shown in FIG. 2, wash liquor anddilution liquor, is led in principle in counter-current to the flow ofpulp via the main conduit 1 and through the bleaching steps of thebleaching line, which flow of pulp (bold arrows are flow lines) passesthrough the sequence W₁-D₀-W₂-EOP-W₃-D₁-W₄-D₂-W₅.

The wash liquor is supplied to the main conduit 1 that is arranged inparallel to the bleaching line, by a pump P20 from a filtrate tank FT₂.According to this embodiment the pump P20 maintains a pressure of about5-6 bar at a first branch portion A1 within the main conduit 1. At thisbranch position A1 wash liquor and dilution liquor (depending on whatkind of wash equipment is being used dilution liquor may possibly bedispensed with. In the case shown in FIG. 2, i.e. using a wash pressboth liquors have to be supplied, which would not be the case if e.g. apressure diffuser was used) is taken in L2 to the subsequent wash W₅ ofthe second bleaching step D₂, from a first branch position A1 in themain conduit. At least a part of the wash filtrate from that wash W₅ ofthe second bleaching step D₂ is then led in L₂ to a second branchposition A2 in the main conduit 1. Wash liquor and dilution liquor istaken via L₃ to the wash W₄ of the first bleaching step D₁, from a thirdbranch position A3 in the main conduit 1, and the filtrate from thiswash W₄ is led via L₄ to a fourth branch position A4 of the mainconduit. Here, the branch positions A1-A4 connect to the main conduitwith the first branch position A1 arranged first, as seen in thedirection of flow in the main conduit 1, and the second to fourth branchpositions A2-A4 in succession thereafter, such that an opencommunication is established in the main conduit between the branchpositions A1-A4. Accordingly, the main pump P20 pressurises the mainliquor within the conduit and establishes a basic flow in the mainconduit in a direction reverse to the formed flow of cellulose pulp inthe bleaching line. Thanks to the pressurisation within the main conduit1 there is no need to use an additional pump to supply wash liquor viaeach respective branch line supplying each respective wash press, seefor example L₁ that supplies W₅ positioned finally in the bleach line.This principle applies to all supply lines L₃, L₅ and L₇, that areconnected to the main line 1. However, for the return lines, L₂, L₄ andL₆, there is a need for a pump P21′, P22′, P23′, to be able to achievesufficient pressure to get it in to the main line again.

By way of example it may be assumed that about 10-12 m³/h is addedthrough each supply line, L₁, L₃, Ls, L₇ to each wash apparatus W₅, W₄,W₃, W₁. Normally about 1-2 m³ of this amount is supplied to the washpress for the actual washing through the upper line, whereas about 9-10m³ is supplied as dilution liquor directly after the wash press W₅.

Due to addition of chemicals in the prior bleaching step D₂ and alsosome fiberlosses, the flow that is taken from the wash apparatus W₅ inreturn through line L₂ back to the main conduit 1 is somewhat largerthan the amount that is supplied through L₁. Accordingly there will be asubsequent addition of liquid flowing in the main conduit in itsdownstream direction. As is evident some of the filtrate from the finalwash apparatus W₅ will be added into the supply line L₃ to the secondlast wash apparatus W₄. As a consequence of the subsequent addition ofliquid/chemicals/fibres the flow adjacent the end of the main conduitwill be approximately about 1-2 m³/h more than is being added at theinlet. Since about 10-12 m³/h is also supplied through the supply lineL₇ at the branch point A7 at the downstream end 10, there will be acontinuous flow of liquid out from the main conduit 1, at the end outlet10 thereof amounting to about 0, 1-2 m³/h (during operation). Theskilled person realises that this example does not limits the scope ofthe invention, but knows that there are many variables, e.g. kind ofwash equipment, production level, kind of bleaching, kind of fibres,etc., that will influence the amount/flow of bleed out.

In FIG. 3 there is shown a further embodiment according to theinvention, wherein a low pressure main conduit 1 is being used (merely aportion of the bleach line is shown since the process is the same asshown in FIG. 2. Accordingly it is evident that the same principles asshown for the upstream portion of the main conduit shown in FIG. 3 alsoapplies for the downstream portion). In such an embodiment a pressure ofabout 1-2 bar is maintained within the main conduit 1. As can been seenin FIG. 3 there is therefore a need to use pumps P21, P22, to pressurisethe wash liquid that has to be supplied to the wash apparatus throughits respective line L₁A, L₃A, etc. However, since the dilution liquid issupplied at an atmospheric addition point, there is no need for using apump for the supply line L₁B and L₃B for that liquid. Accordingly thereis arranged a separate branch position A1′, A3′ for each of those supplylines L₁B, L₃B. Moreover, it is shown that each standpipe SP9, SP7directly subsequent to a bleach tower is supplied via its line L₃C, L₄Cwithout the need for a pump. Also in this embodiment, however a pumpP21′, P22′, is needed to pressurise the filtrate back into the mainconduit 1. Furthermore, there is shown a modification for achieving thedesired pressure in the main conduit, i.e. by replacing the pump with ahigh tower FT2 and level control LC that controls the pressure byregulating the out flow from the outlet (10, not shown) and/or theinflow from L0 to keep the level within the tower FT2 at a desiredlevel. In all other aspects this embodiment is similar to the functionas described in relation to FIG. 2. However, it should be understoodthat the invention may very well be used merely for two bleach steps,e.g. D1 and D2 as shown in FIG. 3.

In FIG. 4 there is schematically shown a part of a bleach line as inFIG. 3, wherein it is presented that filtrate tanks FT3 may be usedwithin a bleach line according to the invention. Moreover it is alsoshown that a pressure buffer tank FT4 may be used in order to balancethe pressure within the main conduit 1. It is evident that a number offiltrate tanks may be used and that they may be of comparatively limitedsize, e.g. less than 1 m³, possibly about 500 litre. As is disclosed thepump P21′ may normally not be dispensed with, since the tank FT₃ ispreferably positioned at a relatively low level and is preferably not apressure vessel. It is also shown that in many cases a further pump P21″is used to supply the filtrate from the wash W₅ to the tank FT₃.

It is, evident that a further main conduit 1′ may suitably be used forbleaching steps of different alkalinity (above or below pH7). Hence,then one main conduit, (e.g. as in FIG. 2) is used for a number ofacidic steps and another main conduit is used for a number of alkalinesteps (the latter not shown).

It is understood that (as shown in FIG. 2) at least one additionalbleaching step D₀ may be provided before the first and second bleachingsteps D₁ and D₂, respectively, as seen in the flow direction of thecellulose pulp, after which additional bleaching step D₀ there is a washapparatus W₂ for the pulp. Then wash liquor and dilution liquor is takento the subsequent wash W₂ of the additional bleaching step, from a fifthbranch position A5 in the main conduit 1. At least a part of the washfiltrate from the subsequent wash W₂ of the additional bleaching step isled to a sixth branch position A6 into the main conduit. The branchpositions connect to the main conduit with the fifth branch position A5arranged after the fourth branch position A4, as seen in the directionof flow in the main conduit 1, and the sixth branch position A6 insuccession thereafter, an open communication being established in themain conduit between the branch positions A1-A6.

An alkaline extraction step, EOP or alternatively an EO step withoutperoxide charge, in a per se conventional manner, is arranged after theadditional bleaching step D₀ and before the first bleaching step D₁, asseen in the direction of flow of the cellulose pulp through thebleaching line, and a wash apparatus W₃ is arranged after the extractionstep EOP. The wash filtrate from the subsequent wash W₃ of theextraction step can be collected in a filtrate tank FT₁ and is suitablyused as dilution liquor before the extraction step and a part of thewash filtrate can if needed be drawn off from the bleaching line, tosewage 11, or be led forward to an oxygen delignification step. As shownin FIG. 2, the cellulose pulp is washed in a wash apparatus W₁ beforethe additional bleaching step D₀, (as seen in the direction of flow ofthe cellulose pulp through the bleaching line) and dilution liquor istaken via L₇ to this wash apparatus W₁ from a seventh branch position A7in the main conduit. The wash liquor to this wash W₁ is taken from aseparate line L₀ as fresh wash liquor.

According to the shown embodiment, at least chlorine dioxide, or someother bleaching chemical that is compatible throughout the bleachingsteps, is used as active bleaching agent in the bleaching steps D₀, D₁and D₂, which chlorine dioxide is added to the pulp before therespective bleaching step in a blending apparatus M1, M3 and M4,respectively.

At the downstream end of the main conduit 1, where the outlet 10 isprovided, from which wash liquor and filtrate is drawn off there isarranged some kind of control device. Preferably, the outlet iscontrolled by a flow controlling control valve FC, which control valvecan establish a certain basic flow and/or a desired bleed-off level offiltrate, during normal operation. The pump P20 is suitable controlledby a pressure regulator PC, enabling feed-back control of the main pumpdevice P20 in order to secure the desired predetermined pressure and/orflow throughout the entire main conduit 1. Suitably, the flowcontrolling valve can establish a desired flow to the outlet 10 as longas the pressure in the main conduit can be maintained. In an alternativeembodiment, the flow controlling valve FC may be a fixed or variablethrottle valve with a high pressure drop over the valve.

According to the invention a bleaching line is provided for thebleaching of cellulose pulp, having at least two bleaching stepscomprising a first and a second bleaching step D₁ and D₂, respectively,as seen in the flow direction of the cellulose pulp, which bleachingsteps have wash apparatuses W₄ and W₅ for the pulp arranged after thefirst and the second bleaching step, respectively, and in which washliquor and where appropriate dilution liquor is led in principle incounter-current to the pulp flow through the wash apparatuses in thebleach line.

The bleaching line preferably also includes at least one additionalbleaching step D₀, which is arranged before the first and secondbleaching steps D₁ and D₂, as seen in the direction of flow of thecellulose pulp. After this additional bleaching step D₀, a washapparatus W₂ for the pulp is arranged. At least one liquor of washliquor and dilution liquor is taken to the subsequent wash W₂ of theadditional bleaching step, from a fifth branch position A5 in the mainconduit 1 and at least a part of the wash filtrate from the subsequentwash of the additional bleaching step is led to a sixth branch positionA6 in the main conduit 1. The branch positions A5-A6 connect to the mainconduit 1 with the fifth branch position A5 arranged after the fourthbranch position A4, as seen in the direction of flow in the mainconduit, and the sixth branch position A6 in succession thereafter, anopen communication being established in the main conduit between thebranch positions A1-A6.

An extraction step may be arranged in the shown bleaching line,preferably of EOP or EO type, which is arranged after the additionalbleaching step D₀ and before the first bleaching step D₁, as seen in thedirection of flow of the cellulose pulp through the bleaching line, anda wash apparatus W₃ is arranged after the extraction step. The washfiltrate from the subsequent wash W₃ of the extraction step is led to afiltrate tank FT₁, via a conduit, and filtrate from the filtrate tankis, at least partly, led as dilution liquor after the wash step W₂subsequent to the additional bleaching step D₀, via pump P30 andconduits, and a part of this wash filtrate is when needed drawn off fromthe process, preferably via an outlet from the filtrate tank FT₁. Asshown in the figure, a part of the liquor in the filtrate tank may alsobe used as dilution liquor in the chute after the EOP reactor.

In the bleaching line, cellulose pulp is suitably washed in a washapparatus W₁ before the additional bleaching step D₀, as seen in thedirection of flow of the cellulose pulp through the bleaching line, andto this wash apparatus W₁ at least one liquor of wash liquor anddilution liquor is led from a seventh branch position A7 in the mainconduit 1 to the wash apparatus W₁, via a pump device P24 and associatedtubing. If the pulp in the storage tower ST is acidic, both wash anddilution liquor in and after the wash W₁, respectively, can be takenfrom the main conduit. But if the pulp in the storage tower is alkaline,a cleaner alkaline filtrate or a clean filtrate is used as wash liquorin the wash apparatus W₁, where the use of a clean filtrate is shown inFIG. 2.

In the bleaching steps D₀, D₁, D₂ of the bleaching line, e.g. chlorinedioxide is charged as active bleaching agent or some other bleachingchemical that is compatible throughout the bleaching steps, such as achelating agent, a pH adjuster or some additional bleaching chemical,which chlorine dioxide or bleaching chemical is added to the pulp beforethe respective bleaching step in a blending apparatus M1, M3 and M4,respectively.

At the end of the main conduit 1, as seen after the branch points A1-A7,an outlet 10 is provided, from which wash liquor and filtrate can bedrawn off from the main conduit. Suitably, the outlet 10 is controlledas is described above, by a pressure and/or flow controlling controlvalve PC and/or FC.

The invention can be varied in a number of ways, within the scope of theclaims. The bleaching steps that in their subsequent wash apparatuseshave a joint main conduit that receives wash filtrate and dilutionand/or wash liquor may, for example, all be of alkaline type or thebleaching chemicals in question may be compatible/blendable. Inmulti-stage bleaching sequences, a main conduit may be used for thealkane filtrate from two or more alkaline steps and another main conduitmay be used for the acidic filtrate from two or more acidic steps.

In the embodiment shown in FIG. 2, the pumps P21-P24 are placed in thefeed conduits from the main conduit. In an alternative embodiment,powerful pumps may be provided in the return conduits that connect tothe branch points A2, A4 and A6, respectively, which in such case,together with the main pump P20, pressurise the entire main conduit.With a pressure in the main conduit established at 4 bar, pumps in thefeed conduits for dilution and/or wash liquor can normally beeliminated. The supply of dilution liquor after wash normally requires avery low pressure of about 1 bar, why a throttle is required for suchdilution liquor supply. Normally, the dilution liquor is supplied to anatmospheric dilution screw in which fluffed-up pulp of high consistency,about 30%, is blended with dilution liquor to a consistency suitable forsubsequent pumping. Therefore, there is a low pressure need on theliquor supply.

In another, alternative embodiment, a basic pressure of about 1 bar maybe established in the main conduit, which is enough to feed dilutionwater, but in which a supplying pump is provided in the feed conduit forthe wash liquor.

Normally, wash liquor is added in a converging wash slot in a wash pressat a higher pressure and normally, a wash liquor pressure of at least2-4 bar is required in this position.

As an additional precautionary measure, a check valve may be providedbetween the branch positions for filtrate recycling to the main conduitand feeding of dilution and/or wash liquor to the wash apparatus inquestion, especially if the branch positions of construction reasons areclose to each other. It is preferred that an open communication isestablished between all branch points in the main conduit, as seen inthe direction from the first end of the main conduit, with the filtratetank FT2, to the second end of the main conduit, with the outlet 10, butthis does not eliminate that valves may be positioned within the mainconduit 1.

Other wash apparatuses than wash presses may of course be used. At lessheavy requirements on chemical carry-over to the subsequent bleachingstep, ordinary filters or simple presses (without washing) may of coursebe used, in which the filtrate from the filter or the simple press isled to the main conduit and optional dilution liquor before the filteror the press is taken from the main conduit. Also, wash presses such asa filter or a simple press without wash, may be connected to a jointmain conduit

The skilled person further realises that the at least two bleach stepsbeing connected to the main conduit 1 may have one or more non-connectedbleach steps, belong in to the same bleach line, in intermediateposition/s between them.

It is also evident that auxiliary equipment, e.g. filtrate tank/s,valves, may be used together with the invention, in certainapplications, despite the fact that such equipment mostly in consideredas superfluors if the invention is used in an optimized manner.Moreover, it is evident that despite the fact that an optimisedembodiment of the invention implies open communication within the wholemain conduit, there may be situations/applications where a part or partsof the main conduit (intermittently or temporarily) may be cut off fromthat communication, e.g. by means of a valve/s. Finally it is understoodthat the extension of the main conduit may vary, e.g. following astraight line and/or being curved and/or having several bends (e.g.90°), etc. to fit different needs at different cites, depending on theposition of items in the bleach line.

It is also evident for the skilled person that the pressurisation atdifferent locations may be achieved by other means than a pump, e.g.instead of pump P20 a tower or the positioning of the main conduit highup may be used to apply the desired pressure (static pressure).

1. A method of bleaching cellulose pulp in a bleaching line, having atleast two bleaching steps comprising: providing a first and a secondbleaching step, as seen in a flow direction of the pulp through thebleaching line, the bleaching steps having wash apparatuses for the pulparranged after the first and the second bleaching steps, respectively,leading wash liquor counter-currently to a pulp flow through thebleaching steps, supplying the wash liquor in a main conduit that ispressurised during steady state, taking the wash liquor to a subsequentwash of the second bleaching step from a first branch position in themain conduit, leading at least a part of the wash filtrate from thesubsequent wash of the second bleaching step to a second branch positionin the main conduit, taking the wash liquor to a subsequent wash of thefirst bleaching step from a third branch position (A3) in the mainconduit, leading at least a part of the wash filtrate from thesubsequent wash of the first bleaching step to a fourth branch position(A4) in the main conduit, arranging the first branch position in a firstposition and the second branch position and the fourth branch positionin subsequent succession relative to the first branch position as seenin the flow direction.
 2. A method according to claim 1 wherein a baselevel of pressure in the main conduit is established at a level in arange of 1,5-3,5 bars.
 3. A method according to claim 2 wherein dilutionand wash liquids taken from the main conduit to dilution vessels or washapparatuses operating at pressures above the base level is pressurisedby a second pressurising means.
 4. A method according to claim 2 whereinthe base level of pressure in the main conduit is established at a levelin a range of 4,5-6,5 bars.
 5. A method according to claim 1 wherein themain conduit is connected to receive and distribute acidic filtrate fromand to, respectively, acidic bleaching steps.
 6. A method according toclaim 1 wherein the main conduit is connected to receive and distributealkaline filtrate from and to, respectively, alkaline bleaching steps.7. A method according to claim 5, wherein upstream the first branchposition (A1) in a first end of the main conduit, a main pressurisingdevice or a pressurised wash liquid tank, is provided which pressurizesthe main conduit and establishes a basic flow in the main conduit in adirection reverse to a formed flow of cellulose pulp in a bleachingline.
 8. A method according to claim 7, wherein before the second andfourth branch positions, filtrate is led to the main conduit via pumpdevices.
 9. A method according to claim 1, wherein at least oneadditional bleaching step is provided before the first and secondbleaching steps, as seen in the flow direction of the pulp, and at leastone liquor of wash liquor and dilution liquor is taken to a subsequentwash of an additional bleaching step, from a fifth branch position (A5)in the main conduit (1) and at least a part of the wash filtrate fromthe subsequent wash of the additional bleaching step is led to a sixthbranch position (A6) in the main conduit.
 10. A method according toclaim 9, wherein an extraction step is provided after the additionalbleaching step and before the first bleaching step, as seen in the flowdirection of the pulp through the bleaching line, and that a washapparatus is arranged after the extraction step.
 11. A method accordingto claim 10, wherein the wash filtrate from the subsequent wash of theextraction step, at least partly is used as dilution liquor for the washstep subsequent to the additional bleaching step, and a part of the washfiltrate is drawn off.
 12. A method according to claim 9, wherein thecellulose pulp is washed in a wash apparatus before the additionalbleaching step, as seen in the flow direction of the pulp through thebleaching line, and at least one liquor of wash liquor and dilutionliquor is taken to the wash apparatus from a seventh branch position inthe main conduit.
 13. A method according to claim 1 wherein chlorinedioxide is used as active bleaching agent in the bleaching steps, whichchlorine dioxide is added to the pulp in a blending apparatus.
 14. Amethod according to claim 1 wherein at an other end of the main conduit,an outlet is provided, from which wash liquor and filtrate are drawnoff.
 15. A method according to claim 14, the outlet is controlled by apressure and/or flow controlling control valve.